In a wireless communication system, a base station may provide one or more coverage areas, such as cells or sectors, in which the base station may serve user equipment devices (UEs), such as cell phones, wirelessly-equipped personal computers or tablets, tracking devices, embedded wireless communication modules, or other devices equipped with wireless communication functionality (whether or not operated by a human user). In general, each coverage area may operate on one or more carriers each spanning a respective frequency bandwidth, and each coverage area may define an air interface providing a downlink for carrying communications from the base station to UEs and an uplink for carrying communications from UEs to the base station. The downlink and uplink may operate on separate carriers or may be time division multiplexed over the same carrier(s). Further, the air interface may define various channels for carrying communications between the base station and UEs. For instance, the air interface may define one or more downlink traffic channels and downlink control channels, and one or more uplink traffic channels and uplink control channels.
In accordance with the Long Term Evolution (LTE) standard of the Universal Mobile Telecommunications System (UMTS), for instance, each coverage area of a base station may operate on one or more carrier bands (or just “carriers”) each spanning 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz. On each such carrier used for downlink communications, the air interface then defines a Physical Downlink Shared Channel (PDSCH) as a primary channel for carrying data from the base station to UEs, and a Physical Downlink Control Channel (PDCCH) for carrying control signaling from the base station to UEs. Further, on each such carrier used for uplink communications, the air interface defines a Physical Uplink Shared Channel (PUSCH) as a primary channel for carrying data from UEs to the base station, and a Physical Uplink Control Channel (PUCCH) for carrying control signaling from UEs to the base station.
In LTE, downlink and uplink air interface resources are mapped in the time domain and in the frequency domain. In the time domain, LTE defines a continuum of 10-millisecond (ms) frames, divided into 1-ms subframes, each made up of two 0.5-ms slots. With this arrangement, each subframe is considered to be a transmission time interval (TTI). Thus, each frame has 10 TTIs, and each TTI has 2 slots. In the frequency domain, resources are divided into groups of 12 sub-carriers. Each sub-carrier is 15 kHz wide, so each group of 12 sub-carriers occupies a 180 kHz bandwidth. The 12 sub-carriers in a group are modulated together, using orthogonal frequency division multiplexing (OFDM), in one OFDM symbol.
LTE further defines a particular grouping of time-domain and frequency-domain resources as a resource block. In the time domain, each resource block has a duration corresponding to one slot (0.5 ms). In the frequency domain, each resource block consists of a group of 12 sub-carriers that are used together to form OFDM symbols. Typically, the 0.5 ms duration of a resource block accommodates 7 OFDM symbols, each spanning 66.7 microseconds, with a 4.69 microsecond guard band (cyclic prefix) added to help avoid inter-symbol interference. Depending on the bandwidth of the carrier, the air interface may support transmission on a number of such resource blocks in the two slots of each TTI. For instance, a 5 MHz carrier supports 25 resource blocks in each slot of each TTI, whereas a 20 MHz carrier supports 100 resource blocks in slot of each TTI.
The smallest unit of resources is the resource element. Each resource element corresponds to one sub-carrier and one OFDM symbol. Thus, a resource block that consists of 12 sub-carriers and 7 OFDM symbols has 84 resource elements. Further, each OFDM symbol and thus each resource element can represent a number of bits, with the number of bits depending on how the data is modulated. For instance, with Quadrature Phase Shift Keying (QPSK) modulation, each modulation symbol may represent 2 bits; with 16 Quadrature Amplitude Modulation (16QAM), each modulation symbol may represent 4 bits; and with 64QAM, each modulation symbol may represent 6 bits.
In some contexts, the two-slot-wide resource blocks of each subframe—and thus each TTI—are considered together as a unit in alternate definition of a resource block. This may be appropriate, for example, when resources are allocated in time in terms of subframes scheduled in TTIs, or when specifying operations and/or configurations that align on TTI boundaries. For example, as described below, downlink control channels are typically configured in first few (typically two) OFDM symbol times of every TTI across all carriers of a carrier band. In the alternate definition, each resource block consists of 12 sub-carriers and 14 OFDM symbols for a total of 168 resource elements.
Within a downlink resource block, and cooperatively across all of the resource blocks of the carrier bandwidth, different resource elements can have different functions. In particular, a certain number of the resource elements (e.g., 8 resource elements distributed throughout the resource block) may be reserved for reference signals used for channel estimation. In addition (and as mentioned above), a certain number of the resource elements (e.g., resource elements in the first one, two, or three OFDM symbols of a subframe-wide resource block) may be reserved for the PDCCH and other control channels (e.g., a physical hybrid automatic repeat request channel (PHICH)), and most of the remaining resource elements (e.g., most of the resource elements in the remaining OFDM symbols) would be left to define the PDSCH.
Across the carrier bandwidth, each TTI of the LTE downlink air interface thus defines a control channel space that generally occupies a certain number of 66.7 microsecond symbol time segments (e.g., the first one, two, or three such symbol time segments), and a PDSCH space that generally occupies the remaining symbol time segments, with certain exceptions for particular resource elements. With this arrangement, in the frequency domain, the control channel space and PDSCH space both span the entire carrier bandwidth. In practice, the control channel space is then treated as being a bandwidth-wide space for carrying control signaling to UEs. Whereas, the PDSCH space is treated as defining discrete 12-subcarrier-wide PDSCH segments corresponding to the sequence of resource block across the carrier bandwidth.
One of the main functions of the PDCCH in LTE is to carry “Downlink Control Information” (DCI) messages to served UEs. LTE defines various types or “formats” of DCI messages, to be used for different purposes, such as to indicate how a UE should receive data in the PDSCH of the current TTI, or how the UE should transmit data on the PUSCH in an upcoming TTI. For instance, a DCI message in a particular TTI may schedule downlink communication of bearer data to a particular UE (i.e., a UE-specific data transmission), by specifying one or more particular PDSCH segments that carry the bearer data in the current TTI, what modulation scheme is used for that downlink transmission, and so forth. And as another example, a DCI message in a particular TTI may indicate the presence of one or more paging messages carried in particular PDSCH segments and may cause certain UEs to read the PDSCH in search of any relevant paging messages.
Each DCI message may span a particular set of TTI-wide resource elements on the PDCCH (e.g., one, two, four, or eight control channel elements (CCEs), each including 36 resource elements) and may include a cyclic redundancy check (CRC) that is masked (scrambled) with an identifier (e.g., a particular radio network temporary identifier (RNTI)). In practice, a UE may monitor the PDCCH in each TTI in search of a DCI message having one or more particular RNTIs. And if the UE finds such a DCI message, the UE may then read that DCI message and proceed as indicated. For instance, if the DCI message schedules downlink communication of bearer data to the UE in particular PDSCH segments of the current TTI, the UE may then read the indicated PDSCH segment(s) of the current TTI to receive that bearer data.
In contrast to the downlink control channel, the LTE uplink control channel (PUCCH) is configured on a select subset of sub-carriers of the carrier band, but across a continuum of TTIs. Typically, the sub-carriers of the PUCCH are allocated like-sized groupings of resource blocks at or near the lower-frequency and upper-frequency band edges of the carrier band. For example, the PUCCH can be configured to occupy the first 12 sub-carriers and the last 12 sub-carriers of the carrier band; this configuration would correspond to two “time-strips” of resource blocks, one strip at each of the lower-frequency and upper-frequency band edges of the carrier band. As another example, the PUCCH can be configured to occupy the first 24 sub-carriers and the last 24 sub-carriers of the carrier band; this configuration would correspond to four “time-strips” of resource blocks, two strips at each of the lower-frequency and upper-frequency band edges of the carrier band.
The PUCCH is allocated in units of resource blocks on a per-UE basis, the number of resource blocks per allocation depending, for example, on how much uplink control channel data each UE needs to transmit. Uplink control channel data from a given UE are transmitted independently of uplink traffic data that the given UE may also need to transmit, and can include such control signaling as HARQ, ACK/NACK, channel quality indicators (QCI), and scheduling requests for uplink traffic transmission, among others.